The present invention relates generally to structures and techniques for filtering radio waves and more particularly to the implementation of a filter network using a combination of superconducting filters and non-superconducting filters.
Radio frequency (RF) equipment have used a variety of approaches and structures for receiving and transmitting radio waves in selected frequency bands. The type of filtering structure used often depends upon the intended use and the specifications for the radio equipment. For example, dielectric filters may be used for filtering electromagnetic energy in the ultra-high frequency (UHF) band, such as those used for cellular communications in the 800+ MHz frequency range. Typically, such filter structures are implemented by coupling a number of dielectric resonator structures together. One can also use metal coaxial resonators in such filters are coupled together via capacitors, inductors, or by apertures in walls separating the resonator structures. The number of resonator structures used for any particular application also depends upon the system specifications and, typically, added performance is realized by increasing the number of intercoupled resonator structures.
However, because of an increase in the number of users utilizing a limited bandwidth, demand has increased for greater frequency selectivity than can be provided by normal or non-superconducting resonator filters, especially for RF signals in the ultra-high frequency bands used for cellular communications. High frequency selectivity has previously been accomplished using High Temperature Superconducting (HTS) filters, usually as front-end filters for cellular base station receivers. However, HTS front-end filters may be susceptible to failure, or degradation in performance, induced by lightning surges or other high power signals. In addition, the non-linearity of HTS filters produces in-band intermodulation spurious signals from out-of-band interferers.
For cellular or similar base stations, typical lightning protectors have only one resonator and do not provide sufficient protection from high power co-located radio frequency signals originating from the transmit side of the base stations. These co-located transmission signals are especially troublesome because they are relatively closely spaced to the operating frequency of the base station receivers. Accordingly, there is a need for a filter that overcomes the above-mentioned and other disadvantages associated with the prior art.
The present invention is directed toward a filter network that provides high frequency selectivity to a receiver. The filter network of the present invention comprises a non-superconducting filter and a superconducting filter. The output of the non-superconducting filter is coupled to the input of a superconducting filter. The non-superconducting filter pre-filters received RF signals by passing RF signals having a frequency within a first pass band to the superconducting filter. The superconducting filter further filters the RF signals to provide a high degree of frequency selectivity at its output.
The filter network of the present invention is able to provide high frequency selectivity while overcoming many of the disadvantages associated with superconducting filters. This is achieved by pre-filtering the RF signals with the non-superconducting filter before inputting them to the superconducting filter. The non-superconducting filter protects the superconducting filter from lightning surges or other high power signals. In addition, the non-superconducting filter filters out interferers that produce in-band intermodulation spurious signals at the superconducting filter output. In a multiplexed configuration, the non-superconducting filter protects the superconducting filter directly from transmit signal energy.
According to one embodiment of the present invention, the non-superconducting resonator filter comprises a housing enclosing three resonators. The resonators are coupled to each other through apertures in the housing. The effect of using this coupling with the three resonators is to produce a filter response with a pass band and a finite frequency transmission zero located outside the pass band.